This invention relates to a disk-like semiconductor wafer retaining structure, and more particularly relates to such a semiconductor wafer retaining structure to prevent impact breakage of the semiconductor wafers during transportation and static electricity caused by friction thereof.
In general, semiconductor wafers (hereinafter simply referred to as wafers) are prepared by cutting a silicon monocrystal ingot into thin disks of, for exsample, about two to eight inches in diameter. The wafers are allowed to apply many circuits such as LSI and cut into chips, which are then packaged to form a semiconductor device.
Each process for cutting such an ingot into wafers, forming circuits on the wafer surface and cutting the wafers into chips to package is often conducted independently at different sites. Generally, a container is used to transfer the wafers from one site of a specific step to the other site. Further, the wafers should be carefully put in the container because electrostatic damage or effect to the wafer surface where circuits are formed would decrease the yield.
In a conventional containing structure, many grooves are formed on an inner wall surface of a specific container so as to put the wafers between such grooves. It is also known to stack each wafer in order through polyethyle films in the specific container so as to form a containing structure.
In the case of the above mentioned first containing structure where many grooves are formed in the inner wall surface of the container, however, mechanical vibration during transportation or accidental drop impact is transmitted to the wafers through the container, thereby the wafers being physically or mechanically damaged. While, in the case of the second containing structure where each wafer is stacked through polyethylene films, the wafers and the films tend to move slightly each other due to mechanical vibration during transportation. Static electricity is generated by such slight movement and charged on the polyethylene films and the wafers, which would damage circuits on the wafers when the electricity is discharged.
Further, when the wafers are put in and out of the container by hand, the wafers would be damaged due to, for example, malfunction. Accordingly, it is preferable to use an automatic control mechanism to put the wafer in and out, but an introduction thereof to conventional containers makes the system more complicated.
It is an object of the present invention to provide a wafer containing structure in which the above mentioned conventional problems are solved so as to prevent a physical and electrical damage of wafers caused by impact during transportation.
A further object of the present invention is to provide a wafer containing structure which can avoid an inconvenient situation where wafers and spacer sheets are united when they are put out of a container because of close and tight contact.
A wafer containing structure of the present invention comprises a container made of a conductive material in which a plurality of semiconductor wafers is stacked and contained, spacer sheets put between the semiconductor wafers and end-cushioning materials put to upper and bottom end portions of a plurality of the semiconductor wafers thus stacked and contained.
The container may be formed by monolithically molding a conductive filler-added conductive plastic material or a polymer alloy-treated conductive plastic material. The conductive filler to be added includes carbon black, graphite carbon, carbon fiber, metal powder, metal fiber, powdery metal oxide, metal-coated inorganic fine powder, organic fine powder and organic fiber, surface resistance of the container being preferably 106 xcexa9/xe2x96xa1 or less.
The spacer sheets are generally made of paper, synthetic paper, synthetic resin films, synthetic resin foam sheets and the like. These spacer sheets may either be electrically conductive or non-conductive. When the spacer sheets are conductive, surface electrical resistance is preferably 106 xcexa9/xe2x96xa1 or less. There may be used polyolefin synthetic paper which contains dispersed conductive fiber such as conductive plastic-conjugated fiber of polypyrrole, polyaniline, carbon fiber and metal-coated fiber; or films or foam sheets of polyethylene, polypropylene, polyethylene phthalate added with conductive fillers, as mentioned above, or antistatic agents.
Although the spacer sheets may either be mono- or multi-layer structure of the above mentioned material such as synthetic paper, the mono-layer structure is preferable from a standpoint of handling and other properties.
Also, synthetic paper of polypyrrole-conjugated fiber is preferable from a standpoint of easy control of conductivity, durability and the like.
The spacer sheets preferably have air permeability of 1,800 sec/100 cc or less according to Japanese Industrial Standard (JIS) P8117, smoothness of 10 sec or less according to JIS 8119 and dust repellency of 200 particles ( greater than 0.5 xcexcm)/100 mmxc3x97100 mm or less according to the Japan CIC Company Standard.
In the containing structure of the present invention, the spacer sheets are put between the wafers (or stacked each other) under a containing condition, as described above.
The container may be vacuum-packed as a whole for a purpose of dust-proof when the wafers are contained in accordance with the containing structure.
In general, as the surfaces of the wafers are extremely smooth, the spacer sheets and the wafers would adhere tightly and come into contact with each other under a practically airless condition.
Because of this, thus adhered wafers and spacer sheets are not easily separated when they are put out of the container in order and it is troublesome to strip off the wafers by hand from the spacer sheets one by one. Particularly, in the case of an automatic control system as will be described bellow, a situation where the wafers and the spacer sheets adhere tightly would result in problems when they are put in and out.
According to the structure of the present invention, it is possible to essentially avoid the above mentioned situation so as to solve conventional problems. Even if the wafers and the spacer sheets adhere to each other, such a trouble can be easily removed by the structure of the present invention.
Each of the spacer sheets may be formed to provide a plurality of concave portions and/or convex portions on at least one surface, or to provide cut-lines extending from a periphery to inside thereof. The concave and/or convex portions may typically be formed by an embossing process. The cut-lines may typically be formed by a cutter or a clicker of certain width.
There may be used soft polyurethane foam, polyethylene foam, polypropylene foam, polystyrene foam and the like as the end-cushioning material. Closed-cell foam is preferable to shut out the dust from outside, while soft polyurethane is desirable from a standpoint of cushioning properties. The cushioning material may be electrically conductive, surface resistance thereof being preferably 1011 xcexa9/xe2x96xa1 or less. When the end-cushioning material is not conductive, it is preferable to arrange the conductive spacer sheets at the uppermost wafer and the bottom one.
According to the containing structure of the present invention, each end-cushioning material is put on the top end portion and under the bottom end portion of the spacer sheets and the wafers stacked one after the other to contain them in the container.
It is preferable to use a cushioning material having 10%-compressive stress of about 0.01 to 0.6 kg/cm2. For example, polyurethane foam of about 0.01 to 0.03, polyethylene foam of about 0.2 to 0.4, and polystyrene foam of about 0.2 to 0.4 kg/cm2 are preferably used.
In the above mentioned wafer container structure of the present invention, the wafers and the spacer sheets are stacked one after the other and contained in the container with the end-cushioning materials put on the top and under the bottom ends. Accordingly, vibratory impact to the wafer container during transportation can be absorbed by the end-cushioning materials, thereby protecting the wafers from such impact.
When a conductive material is used as the spacer sheets, no static electricity generates if friction is created between the wafers and the spacer sheets or between the wafers and the end-cushioning material. Even if the static electricity happens to generate, such electricity does not remain in the container. As a result, any circuit formed on the wafer never be inconveniently damaged by the static electricity.
Further, each wafer can be stably contained in the container without any mutual displacement thereof because the spacer sheets and the wafers are stacked one after the other and retained from both of the upper and the bottom directions in the container. Thus, contact breakage of the wafers to the inner surface of the container or static electricity caused by friction of mutual wafer can be controlled more effectively.
The surface resistance of the container is preferably 106 xcexa9/xe2x96xa1 or less so as to rapidly release static electricity generated in the container or to prevent an effect of external electric field.
In the containing structure of the present invention, the container is provided with the cylindrical portion for putting the wafers therein, which may have slits to receive pickup arms for putting the wafers or the spacer sheets in and/or out thereof. These slits make it possible to easily put the wafers and the conductive sheets in and/or out of the container.
According to the present invention, there is provided a method for putting the wafers in and/or out of the container in which the wafers and the spacer sheets are put in and/or out of a cylindrical portion of a container main body, the cylindrical portion having at least one slit thereon, by means of one or two pickup arms. The present method is adaptable to an automatic controlling mechanism.
When one pickup arm is used, the arm enters into at least one slit and put each wafer and spacer sheet in the cylinder portion or put out therefrom.
When two pickup arms are used, one of the two arms enters into the slit, while the other one enters into the same or another slit. Each of the wafers and spacer sheets is put in or put out of the cylinder portion by means of these two pickup arms.
According to the present method for putting the wafers in and/or out of the container, the container provided with the slitted cylinder is used, which makes a head portion of the pickup arm to enter into inner portion of the cylinder when the wafers are put in and/or out of the cylinder of a container. Accordingly, if the number of the wafers increases gradually when they are put in, or if number thereof decreases gradually when they are put out, control of the pickup arm is quite easy in an automatic control mechanism.